Three patients each have a homonymous hemianopia: one from an optic tract lesion, one from a temporal lobe infarct, and one from a posterior cerebral artery occipital infarct. Which lesion produces the MOST congruous (most closely matching between the two eyes) field defect?

• A Optic tract
• B Occipital cortex ✓
• C Temporal lobe (Meyer's loop)
• D Congruity is identical at all three sites

Explanation

Congruity increases progressively from anterior to posterior along the visual pathway because retinal correspondence becomes more precise as fibres consolidate. Occipital cortical lesions give highly congruous, often precisely quadrantic homonymous defects, while optic tract lesions are notoriously incongruous. Meyer's loop lesions are intermediate and typically incongruous superior quadrantic defects, making option C the tempting but wrong choice.

Reference: Kanski's Clinical Ophthalmology, 9th ed.

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